1. Field of the Invention
The present invention relates to a method for manufacturing a lanthanum oxide compound which can exhibit satisfactory electrical insulation and high dielectric constant.
2. Description of the Related Art
As of now, oxide silicon films or oxynitride silicons would be employed as gate insulating films of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) in a semiconductor integrated circuit or insulating films for electrodes of memory cells in a FLUSH memory. With the recent requirement of the downsizing of the MOSFET and the memory cell, it is also intensely required to reduce the thickness of the gate insulating film and the thickness of the insulating film (e.g., blocking oxide). In this point of view, it may be that the oxide silicon film and the oxyitride silicon film can not maintain the insulation requiring for the gate insulating film and the insulating film. Therefore, such an attempt is made as replacing the conventional oxide silicon film and oxynitride silicon film with a new insulating material which can be reduced in thickness while the electrical insulation can be maintained sufficiently. Since lanthanum oxide compound such as lanthanum aluminate (LAO) and lanthanum hafnate (LHO) has high dielectric constant and large bandgap, the lanthanum oxide is promising as the newly replacing material (Reference 1).
In order to utilize the lanthanum oxide industrially, it is required to develop a new film forming method and a film forming apparatus which can realize mass production. Generally, some CVD (Chemical Vapor Deposition) methods would be employed in the film forming steps in the production line of a semiconductor manufacturing process. However, since the vapor pressure of the lanthanum is very low, it may be difficult to form the lanthanum oxide compound film by means of CVD method. In this point of view, it is considered that the lanthanum oxide compound film is preferably formed by means of PVD (Physical Vapor Deposition) method instead of the CVD method (Reference 2).
An MBE (Molecular Beam Epitaxy) method can be exemplified as the PVD method. The MBE can exhibit good performances such as the easy controllability in supplying rate of raw materials and low damage for a substrate during the supply of the raw materials in comparison with a sputtering method and the like.
In the case that an oxide film is formed by means of the MBE method, a metal raw material and an oxygen raw material are supplied simultaneously. For example, the lanthanum oxide (LAO) compound film can be formed by supplying an aluminum raw material, a lanthanum raw material and an oxygen raw material simultaneously for the intended substrate. However, there are some problems in the formation of the lanthanum oxide compound film by means of the MBE method as follows.
In the case that the supplying rate of the lanthanum raw material is so high that the intended oxidation cannot be realized, some oxide defects may be generated in the resultant lanthanum oxide compound film. In contrast, it is desired to increase the supplying rate of the lanthanum raw material in mass production while the oxygen defects of the lanthanum oxide compound film may deteriorate the inherent electrical insulation thereof. As a result, the lanthanum oxide film may not be formed so as to realize the electrical insulation as designed initially.
[Reference 1] J. Appl. Phys. Vol. 76, No. 9, 1006 (2007), Yasuo Nara
[Reference 2] U.S. Pat. No. 6,770,923